Filtering Mail with Milter
David F. Skoll
Roaring Penguin Software Inc.
Overview
Why filter mail?
Different filtering approaches
Delivery agent (e.g. Procmail)
Central filtering (Milter)
Milter Architecture
Milter API
Pros and cons of Milter API
MIMEDefang Architecture
Conclusions
Why Filter Mail?
The old reason: to stop viruses.
The new reason: to stop spam and
inappropriate content.
Blocking viruses is easy. Block .exe and
friends, and test against signature
databases.
Blocking spam is hard, but becoming
increasingly important. Organizations can
even face lawsuits over inappropriate
content.
One-slide Overview of Mail
Delivery
Mail
Store
MUA
SMTP
MTA
LDA
SMTP
MTA
SMTP
POP/
IMAP
MUA
(Procmail)
MTA
MUA = Mail User Agent. What you use to send and read mail. Eg: Mozilla, Evolution
MTA = Mail Transfer Agent. Eg: Sendmail, Postfix, Exim, qmail
Mail Store = Where mail is stored until it is read. Usually in /var/spool/mail
LDA = Local Delivery Agent. Program which performs final delivery into the mail store
POP = Post Office Protocol. A method for retrieving mail from a mail store
IMAP = Internet Message Access Protocol. A better protocol than POP/POP3.
SMTP = Simple Mail Transfer Protocol. The language used by MTA's to talk to each other.
Filtering with Procmail
The local delivery agent does the filtering.
PRO: Easy to customize filter rules perrecipient. Procmail well-known and tested.
CON: Inefficient. Messages sent to multiple
users are processed multiple times.
CON: Local users only. Difficult to use
procmail on a store-and-forward relay.
CON: SMTP transaction is complete by the
time Procmail is invoked.
Central Filtering (Topic of This Talk)
The mail transfer agent does the filtering.
PRO: Efficient. Each message filtered once,
regardless of number of recipients.
PRO: Can modify the SMTP dialog.
Amazingly useful, as you'll see...
PRO: Can filter relayed mail, not just local.
CON: Harder (but not impossible) to
implement per-recipient filter rules.
Milter
Milter (Mail fILTER) is both a protocol and a
library.
Milter was designed by the Sendmail
development team, and has been part of
Sendmail since 8.10. Really matured in 8.12.
Milter lets filters “listen in” to the SMTP
conversation and modify SMTP responses.
Milter is extremely powerful and can let you
do amazing things with e-mail.
Milter Architecture
TCP or UNIX-domain Socket
Sendmail Daemon
Milter 1
Sendmail Daemon
Milter 2
Sendmail Daemon
Milter N
Sendmail is single-threaded, but forks into multiple processes. The milter library
is multi-threaded, and a given Sendmail installation can have multiple mail filters
(filters 1 through N above.)
Milter API Operation
At various stages of the SMTP conversation,
Sendmail sends a message over the socket
to the milter.
The milter library invokes a callback into your
code. It then sends a reply message back to
Sendmail containing the return value from
your callback.
In addition, you can call milter library
functions that send special messages to
Sendmail to modify aspects of the message.
Typical SMTP Conversation
C:
S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
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C:
S:
Connect to server
220 server_hostname ESMTP Sendmail 8.12.7/8.12.7...
HELO client_hostname
250 server_hostname Hello client_hostname, pleased...
MAIL FROM:<dfs@roaringpenguin.com>
250 2.1.0 <dfs@roaringpenguin.com>... Sender ok
RCPT TO:<foo@roaringpenguin.com>
250 2.1.5 <foo@roaringpenguin.com>... Recipient ok
RCPT TO:<bar@roaringpenguin.com>
250 2.1.5 <bar@roaringpenguin.com>... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
(transmits message followed by dot)
250 2.0.0 h0AJVcGM007686 Message accepted for delivery
QUIT
221 2.0.0 server_hostname closing connection
Typical SMTP Conversation with
Milter
C:
S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
S:
Connect to server
220 server_hostname ESMTP Sendmail 8.12.7/8.12.7...
HELO client_hostname
250 server_hostname Hello client_hostname, pleased...
MAIL FROM:<dfs@roaringpenguin.com>
250 2.1.0 <dfs@roaringpenguin.com>... Sender ok
RCPT TO:<foo@roaringpenguin.com>
250 2.1.5 <foo@roaringpenguin.com>... Recipient ok
RCPT TO:<bar@roaringpenguin.com>
250 2.1.5 <bar@roaringpenguin.com>... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
(transmits message followed by dot)
250 2.0.0 h0AJVcGM007686 Message accepted for delivery
QUIT
221 2.0.0 server_hostname closing connection
*= response-modification opportunity *= filtering opportunity
*
*
*
*
*
*
Milter API Functions
Initialization:
smfi_register
Register a filter
smfi_setconn
Specify socket
smfi_settimeout
Set timeout
smfi_main
Enter main milter loop
You “register” a filter and tell what kinds of
callbacks you're interested in, and whether or
not you might modify the message headers
and body.
Milter API Functions: Data Access
smfi_getsymval Get value of a Sendmail macro
smfi_getpriv
Get arbitrary private data
smfi_setpriv
Set arbitrary private data
smfi_setreply
Set SMTP reply code
Accessing Sendmail macros lets you access
a lot of useful info.
Private data useful for storing thread-specific
data.
Can set SMTP reply to anything you like
Milter API: Message Modification
smfi_addheader
Add a header
smfi_chgheader
Change a header
smfi_addrcpt
Add a recipient
smfi_delrcpt
Delete a recipient
smfi_replacebody Replace message body
Recipient functions affect envelope
recipients, not headers.
Milter API: Callbacks
xxfi_connect
xxfi_helo
xxfi_envfrom
xxfi_envrcpt
xxfi_header
xxfi_eoh
xxfi_body
xxfi_eom
xxfi_abort
xxfi_close
Called when connection made
Called after HELO
Called after MAIL FROM:
Called after RCPT TO:
Called for each header
Called at end of all headers
Called for each “body block”
Called at end of message
Called if message aborted
Called when connection closed
Callback Types
Connection oriented: Apply to connection as
a whole. xxfi_connect, xxfi_helo,
xxfi_close.
Recipient oriented: Apply to a single recipient
only. xxfi_envrcpt
Message-oriented: Apply to a message. All
other callbacks are message-oriented.
Callback Return Values
SMFIS_CONTINUE: Continue processing
SMFIS_REJECT: For connection-oriented callback,
close the whole connection. For message-oriented,
reject the message. For recipient-oriented, reject
only this recipient.
SMFIS_DISCARD: For message- or recipientoriented routine, silently discard message.
SMFIS_ACCEPT: Accept without further filtering.
SMFIS_TEMPFAIL: Return a temporary-failure
code for recipient, message or connection.
Pros and Cons of Milter API
Pro: Sendmail's implementation is not too buggy.
Pro: Quite efficient. Filters do what they need and
no more.
Pro: Can modify SMTP return codes.
Con: Written in C. C is not the best language for
parsing e-mail messages. Hard for end-users to
write filters.
Con: Multi-threaded. Easy to make dumb
programming errors; some pthreads
implementations leave a lot to be desired.
MIMEDefang
MIMEDefang is a GPL'd, Perl-based mail
filter. It uses Milter to interface with
Sendmail.
Currently used in thousands of different sites.
Larger MIMEDefang installations process >1
million e-mail messages per day.
MIMEDefang runs on Linux, FreeBSD,
Solaris, Tru64 UNIX, HP-UX, AIX, ...
http://www.roaringpenguin.com/mimedefang/
MIMEDefang Architecture
mimedefang
UNIX-domain socket
mimedefang-multiplexor
Pipes
mimedefang.pl
mimedefang.pl
mimedefang.pl
Multithreaded mimedefang threads talk to single-threaded
mimedefang-multiplexor
Multiplexor manages a pool of single-threaded Perl slaves that do
the actual filtering.
MIMEDefang Advantages
Robust: Perl slaves are killed after they
process a certain number of messages. Perl
memory leaks are therefore not a problem.
Efficient: Idle Perl processes are fed work.
No per-message Perl startup overhead.
Similar to Apache's pre-forked MPM.
Scalable: Just add processors and RAM...
Easy to use: Single-threaded Perl filters
much easier to write than multi-threaded C.
Can take advantage of CPAN goodies.
MIMEDefang Advantages – 2
Lots of built-in functionality:
Hooks to anti-virus software
Hooks to SpamAssassin
Easy to drop attachments or reject mail based
on filenames (*.exe, for example)
Can add/delete recipients, add/delete/change
headers.
Almost all Milter functionality is exposed to
MIMEDefang.
MIMEDefang in the Real World
Large US financial institution filters >1 million
messages/day on a couple of Sun E-450's.
Large US PC manufacturer filters > 1.5
million messages/day on 10 Linux PC's.
One crazy MIMEDefang list member reported
filtering almost 2 million messages/day on a
single specially-tuned Intel/Linux box. (He
modified the Perl filter quite extensively.)
Example MIMEDefang Filter
Block Korean mail (filter fragment):
$head = $entity->head;
$charset = $head->mime_attr("content-type.charset");
if (defined($charset)) {
$charset =~ tr/A-Z/a-z/;
if ($charset eq "ks_c_5601-1987" or
$charset eq "euc-kr") {
return action_bounce(
"Spam from Korea not accepted.");
}
}
Example MIMEDefang Filter 2
Tempfail open relays and disbelieve
spamware that uses our domain in HELO
sub filter_relay ($$$) {
my ($ip, $name, $helo) = @_;
if ($helo =~ /roaringpenguin\.com$/i) {
return('REJECT', 'Liar... not roaringpenguin');
}
if (relay_is_blacklisted($ip, 'ordb.org')) {
return('TEMPFAIL',
'Try again when you are no longer an open relay');
}
return('CONTINUE', 'OK');
}
More on MIMEDefang
Perl callbacks are called after MAIL FROM:,
RCPT TO: and end of DATA phase.
All Sendmail macro values are accessible.
Can do things like:
Replace large attachments with URLs
Omit spam-scanning for users
authenticated with SMTP AUTH
Quarantine viruses
Make copies of incoming/outgoing mail
CanIt
CanIt and CanIt-PRO are commercial (I.E.,
not open-source) solutions built around
MIMEDefang.
Features include:
Sophisticated SQL-based spam trap.
Lots of anti-spam tricks.
Simple Web interface for managing spam.
Per-recipient traps (CanIt-PRO) with
individual rules, thresholds, etc.
CanIt
CanIt is not open-source because I have to
eat. (You get the source, but can't
redistribute it. You can modify it for internal
use.)
However, CanIt revenues support continuing
development of MIMEDefang, and some
CanIt features eventually migrate their way
into MIMEDefang.
CanIt URL: http://www.canit.ca/
Conclusions
Filtering at the SMTP server is the best place
to filter mail.
More efficient than LDA approaches.
More flexible --- can modify SMTP return
codes.
With a bit of cleverness, you can still have
per-recipient rules.
Sendmail's Milter API is a pretty decent API
for achieving robust server-based mail
filtering.